I've got a Seagate slimline 3.5" inside the 1200... it doesn't make a bump or anything, it fits just tight. You could try and catch one of those if you want.
I am looking forward to put it in an external case, though. How long can an IDE cable be? I am thinking about running the cable to the outside through the unused slot on the back (I have no Blizzard or GVP accel, so no SCSI interface for me :/) but I feel the cable I can use might be a bit short and I won't be able to do this.